This invention relates to a method for treating BOD-containing water by oxygenation. The BOD-containing water may for example be municipal waste, chemical waste from petrochemical or paper plants, or fermentation liquor.
Biochemical oxidation methods employ aerobic bacteria to convert various substrates and nutrients to other forms of matter. A common example is the activated sludge method for purifying sewage and industrial wastes. In such methods, the type and rate of reactions which occur are critically dependent upon the presence of ample oxygen for use by the bacteria. The oxygen is made available to the bacteria by dissolution into the liquor from an aerating gas, and by uptake of the dissolved oxygen (DO) by the bacteria.
High DO levels in the liquor are desirable for several reasons. For example, anaerobic zones are avoided and the rate of the biochemical reaction is not hindered from a lack of oxygen. Moreover, the bacterial population is improved by high DO levels because the growth of anaerobic and facultative strains is suppressed. Such strains cause odors and extend treatment time. Under certain conditions including high DO, a bacterial floc is formed which settles more rapidly to higher densities. This produces an improved effluent and renders the BOD-containing water treatment system less susceptible to upsets. Another desirable characteristic is that the large, desirable floc particles are more adequately supplied with oxygen throughout their mass because the DO gradient supplied through the particle is higher. Finally, high DO in the liquor in the system means that higher solids levels can be sustained with resultant higher treatment rate and lower production of excess sludge.
Air is the common source-gas for dissolution of oxygen into the liquor. A common dissolution technique is to sparge or diffuse compressed air into lower levels of open treatment tanks filled with a mixture of liquid and bacterial solids (mixed liquor). The sparge air serves the dual purposes of creating a large gas-liquor interfacial area for dissolution of oxygen, and of stirring the mixed liquor so that the solids remain in uniform suspension. For municipal sewage treatment, about 500 to 700 cu. ft. of air is usually diffused per lb. BOD removed from the influent water, and with 4 to 8 hours solids retention time, this corresponds to about 110-150 cu. ft. air per hour per 1000-gallon aeration tank capacity. Of the oxygen contained in this air, about 10% is dissolved and utilized in the biochemical oxidation and the remaninder is wasted. The amount of air which needs to be introduced solely to keep the solids in suspension is on the order of 70-80 c.f.h. per 1000-gallon tank capacity, and is substantially less than that actually introduced in order to dissolve sufficient oxygen. Hence, it is seen that the amount of air supplied is dictated by "oxygen demand," and the amount of air is very large because of the low fraction of its contained oxygen which can be dissolved. The air is compressed to a level determined by friction in the system and submergence of the diffusers (e.g., 10 p.s.i.g.). Power costs vary between about 0.25 and 1.6 k.w.h. per lb. BOD removed, and average about 0.56 k.w.h. per lb. BOD removed.
It has long been recognized that the use of air as an oxygen source imposes a serious limitation on the rate of oxygen dissolution which can be sustained. Air contains only 20.8% oxygen and its other constituents are inert to the biochemical reactions. In practice, the dissolved oxygen is consumed from the mixed liquor by the bacteria so rapidly that the DO levels economically achievable with air aeration are suppressed below safe levels for a healthy, profuse growth of desirable aerobic bacteria. Anaerobic and faculative strains of bacteria may develop which cause odors and extend treatment time.
High solids levels in the aeration zone are also beneficial to BOD-containing water aeration because the rate of BOD removal becomes higher and the rate of excess sludge production becomes lower. However, high solids levels result in more rapid uptake of DO by the biomass, and in deference to the limited rate of dissolution of oxygen from air, waste treatment practitioners have deliberately reduced and suppressed active solids levels in the mixed liquor. When the solids level is reduced, the rate of BOD removal is decreased and treatment tanks remain large in order to retain the waste for the requisite time period (3-6 hours) necessary for purification.
The rate of oxygen dissolution can be increased by more violent agitation of the body of mixed liquor using surface aerators, beaters and submerged turbines. However, severe agitation breaks up and disperses the flocculant agglomerates so that after treatment, the solids do not separate properly from the effluent. Moreover, the solids, when gravity-settled, possess a high specific volume (Mohlman SVI) and the necessary recycle of such solids as inoculant represents a severe hydraulic burden on the system. Under conditions of low DO and low solids levels, the floc particles are small and fragile and are particularly susceptible to dispersion by attrition. Attempts to "overpower" the aeration system have also led to prohibitive investment and operating costs of the equipment involved.
It can be seen that the use of air as an oxygen source for biochemical reactions imposes serious penalties on the method. It has been proposed to use pure oxygen or oxygen-enriched air for aeration as a means of increasing dissolution rates. With pure oxygen, it is possible to increase the oxygen partial pressure difference between gas and liquid by five-fold. Many attempts have been made to utilize oxygen-enriched aeration gas but without commercial success. In some of these attempts, the oxygen-enriched gas was merely substituted for air using equipment and procedures common to air aeration. The high cost and low economy of these efforts resulted from the ineffective utilization of the oxygen which unlike air, is not "free" from the atmosphere. For example, when pure oxygen is sparged or diffused in the normal way into a conventional treatment tank for municipal sewage, only about 5-10% of the oxygen is consumed (i.e. Dissolved and utilized) and the remainder escapes to the atmosphere.
One of the best known of the prior art attempts to employ oxygen-enriched aeration gas is the bioprecipitation process wherein a fraction of the effluent from a combined reactor-clarifier is mixed with the influent, oxygenated to near saturation, and then returned to the base of the reactor. The reactor contains a blanket of active solids and the highly oxygenated liquid rises slowly through the blanket, thereby transferring its organic pollutants to the bacterial floc and also supplying the needed oxygen for assimilation. The influent plus recycle effluent is oxygenated by downflow through a countercurrent gas-liquid contacting column--the oxygen-rich aeration gas being introduced at the base of the column and vented at the top. Although the countercurrent contactor is probably the most efficient mass transfer device for most chemical processes, it too has failed to achieve the economy necessary for oxygen aeration of a biochemical oxidation process. Only 20 to 25% consumption of the feed oxygen has been realized.
One reason for the low utilization of the bioprecipitation process is the very fact that the DO level in the oxygenator or contactor was pushed to near saturation with the result that the oxygen partial pressure driving force essentially vanished in the lower levels of the colume. Simultaneously, the CO.sub.2 and N.sub.2 impurities stripped from the liquid severely depressed the oxygen partial pressure driving force in upper levels of the column. These combined factors prevented dissolution of an economical, high fraction of the oxygen introduced. The near approach to DO saturation is a necessary objective of the process, since the full DO supply to treat the BOD-containing water must be contained and "carried" in the flow of diluted influent to the reactor.
It is an object of this invention to provide an improved method for treating BOD-containing water by oxygen-enriched aeration gas.
Another object is to provide a method characterized by relatively high consumption of oxygen in the aeration gas.
Still another object is to provide a method characterized by relatively high oxygen consumption, high dissolved oxygen in the mixed liquor, and high solids concentration.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.